refactor: switch wal to sync implementation

[bsbds]

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

  Switch the curp WAL to synchronous implementation, as the current async version is slow in benchmarks.

• what changes does this pull request make?

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)